1. Field of the Invention
The present invention relates to a motor in which a bearing set is composed of a bearing and a housing for holding the bearing, or a bearing set is composed simply of a bearing and, in particular, to an improved fixing structure for fixing such a bearing set to a circuit board.
2. Description of the Related Art
For example, as a brushless motor which is used for a disk drive, there is known a structure which is shown in FIG. 12. A circuit board 31 includes a plurality of parts (not shown) mounted therein so as to make a desired circuit. The circuit board 31 includes a through-hole 41 formed in the central portion thereof and a screw hole 32 formed in the radial direction thereof. In the through-hole 41, there is mounted a cylindrical housing 35 which holds a bearing 34 with the inner peripheral surface thereof and includes a flange portion 36 formed on the outer peripheral surface thereof. Also, a rotary shaft 37 to be rotatably supported by the bearing 34 is inserted into the through-hole 41. In the flange portion 36 of the housing 35, there is formed a screw hole 42 such that it coincides with the screw hole 32, and the lower surface of the flange portion 36 is in contact with the upper surface of the circuit board 31. As the material of the housing 35, generally, there is used a relatively soft material such as a zinc die casting, an aluminum die casting, a resin molding or the like. Here, the lower surface of the flange portion 36 of the housing 35 is race-worked in order to enhance the mounting accuracy of the housing 35 when it is fixed to the circuit board 31, which will be described later in detail.
A stator core 39 including a screw hole 43 is in contact with the upper surface of the flange portion 36 of the housing 35 in such a manner that the screw hole 43 coincides with the screw hole 42 formed in the flange portion 36. If a screw 44 is so inserted into the screw hole 43 as to extend through the screw hole 43 and thus the screw 44 is screwed into the screw hole 32 in the circuit board 31, then the flange portion 36 of the housing 35 is pressed by the screw 44 so that the housing 35 can be fixed to the circuit board 31. The stator core 39 is formed of a plurality of core materials superimposed on top of one another, and includes a plurality of salient poles around which a stator coil 40 is wound. Here, the bearing 34 and housing 35 form a bearing set 33, the stator core 39 with the stator coil 40 wound therearound forms a core winding set 38, and a stator set 45 can be formed by the bearing set 33 when it is mounted to the circuit board 31 by the core winding set 38.
A cup-shaped rotor case 46 is disposed on the upper portion of the stator set 45. A drive magnet 47 is fixed to the inner peripheral surface of the rotor case 46 in such a manner that the drive magnet 47 is opposed to the salient poles of the stator core 39. In addition, the rotor case 46 includes on the upper surface thereof a hub base 48 on which a disk can be placed. Also, the rotary shaft 37, which is to be inserted into the through-hole 41 in the circuit board 31, is fixed to the central portion of rotor case 46. The rotor case 46, which includes the drive magnet 47 and hub base 48, and the rotary shaft 37 cooperate in forming a rotor set 49. In the motor having the above-mentioned structure, if the stator coil 40 of the stator core 39 is energized electrically, then the energizing current and a magnetic field generated from the drive magnet 47 interact with each other, so that the rotor set 49 starts to rotate as an integral body.
However, in the above-mentioned conventional motor, there remains the following problems to be solved:
(1) Since the housing 35 of the bearing set 33 is formed of a soft material, when trying to make it thinner, the thickness of the flange portion 36 thereof is relatively smaller than the thickness of the stator core 39. On the other hand, when the stator core 39 is pressed by the screw 44, the pressing force of the screw 44 is concentrated onto one point to thereby deform the surface of the stator core 39. For these reasons, even if the mounting accuracy of the mounting surface of the flange portion 36 of the housing 35 with respect to the surface of the circuit board 31 is enhanced, when the flange portion 36 is inserted between the stator core 39 and the circuit board 31 and is screwed there, the material of the housing 35 is absorbed into the deformed surface of the stator core 39, so that the assembling accuracy of the housing 35 with respect to the circuit board 31 is worsened.
(b 2) It is necessary to race-work the mounting surface of the flange portion 36 of the housing 35 with respect to the circuit board 31 and it is also necessary to work the screw hole 42. This increases the number of working steps, which in turn increases the cost of the motor.
(3) Because the core winding set 38 is assembled to the circuit board 31 after the bearing set 33 is assembled, a reflow soldering method cannot be used, which results in the increased cost and in the unstabilized quality of the motor.